This invention relates to printing presses, and, more particularly, to a deck configuration for a printing press.
Printing presses such as flexographic presses include one or more decks for supporting rolls adjacent a central impression (CI) drum or cylinder. For example, a flexographic press typically includes multiple color decks, and each color deck includes a plate roll and an anilox roll. The anilox roll transfers ink from an ink fountain to the plate roll. The plate roll carries the print image and imprints the image onto a web which is supported by the central impression (CI) drum. The plate roll and anilox roll of each deck are mounted for movement toward (racked in position) and away (racked out position) from the CI drum.
A deck design which is currently being offered by Paper Converting Machine Company, the assignee of this invention, uses a set of linear bearings mounted on a linear rail on each of the front and rear frames of the press. The plate and anilox rolls are mounted on top of the linear bearings and are moved along the linear rails by a ball screw.
Forces which are generated in the printing nip between the CI drum and the plate roll are transferred to the printing deck through the bearings which support the journals of the plate and anilox rolls. The configuration results in a moment load byproduct that must be absorbed by the linear bearings as the ball screw absorbs the linear forces from the printing nip. Linear bearings, although capable of supporting modest moment loads, are generally not intended for robust support of moment loads. The net effect is that relatively significant deflections can be incurred from modest nip forces due to the linear bearings rocking on the linear rails.
As the line of action of the axis of the plate and/or anilox rolls moves away from the rotational axis of the CI drum, the nip forces transmitted to the deck have a higher impact on the deflection of the deck components. These deflections ultimately compromise the quality of the printing that can be achieved, primarily from susceptibility to bounce.
The problem with the present art is the moments that are applied to the color deck components because of their configuration. In the present configuration, the ball screw and plate/anilox rolls are not mounted in-line with each other. During normal printing conditions, a force is applied to the plate/anilox rolls and is then transmitted to the other color deck components. The ball screw is the only component that resists a horizontal load, so a moment is created because the plate/anilox rolls are mounted above the ball screw. The larger the vertical distance between the ball screw and the plate/anilox rolls, the larger the moment.
The linear bearings are the only component that can resist the moment load, but because of the limited distance the runner blocks can be spaced apart, high moment loads result in large plate/anilox roll deflections. The moment loads are undesirable for horizontal decks and are amplified for angled decks. As a result of the moments applied due to the prior art configuration, all of the color deck components rotate as printing forces are applied. The net result from this deck design is lowered deck stiffness (spring elements in series) and higher susceptibility to bounce during printing.